In the related art, techniques for controlling operations of various safety devices according to a weight of a passenger sitting in a seat are known to improve performances of the safety devices equipped in a vehicle such as seat belts and airbags. For example, it is typical that a warning light of “Fasten Seat Belt” is displayed after a detection of the sitting when the passenger sits in the seat but does not fasten the seat belt. In addition, according to regulations in practice in North America, the airbag has to be deployed in case of an accident in a case where an adult sits in an assistant driver's seat. In this manner, it is very important, in terms of safety, to detect and accurately determine the weight of the passenger.
As an example of the related art, JP 2003-83707A (Reference 1) discloses a strain sensor that includes a sensor substrate (strain body) which has two fixed holes fixed to one member disposed in both end portions of a long plate, a detection hole arranged between the two fixed holes to be fixed to the other member, and at least one strain detection element adhered to an upper surface of the long plate between the fixed holes and the detection hole, the strain sensor further including a fixing member that is press-fitted into each of the fixed holes of the sensor substrate, and a detection member (connection member) that is press-fitted into the detection hole of the sensor substrate. An external force is applied to the end portion of the detection member abutting against the sensor substrate and a reaction force is applied to the end portions of the two fixing members abutting against the sensor substrate, and a strain is detected by using the strain detection element adhered to the sensor substrate based on the external force and the reaction force.
In the strain sensor according to JP 2003-83707A (Reference 1), a plurality of strain detection elements are arranged in parallel on a central straight line in a longitudinal direction of the sensor substrate (strain body). These strain detection elements can detect a load value or the like by using the strain caused by pulling and compression with respect to one direction (direction parallel with a gauge length). Accordingly, in a case where a load of the passenger in an up-down direction added to the vehicle seat is transmitted to the sensor substrate, the weight of the passenger can be detected by using the strain of the strain detection element arranged in the sensor substrate when the strain sensor according to JP 2003-83707A (Reference 1) is interposed between the vehicle seat and the floor side fixed member, the detection hole is connected to a seat frame (seat side fixing member), and the fixed hole is connected to the floor side fixed member.
The seat frame in which the sensor substrate is arranged is a rigid body that has a rectangular shape. However, due to a biased load applied to the seat frame, both ends of the seat frame in a left-right direction tend to be obliquely deformed in opposite directions about an axis of rotation (imaginary) in the left-right direction.
Accordingly, a relative angular displacement may be generated between the floor side fixed member and the seat frame about the axis of rotation in the left-right direction. The relative angular displacement affects a strain of the strain detection element performing the weight detection, and thus the weight of the passenger may not be detected with accuracy.